The airfoils or profiles used in wind turbine blades have different performance characteristics in the root region, the mid-span region and the tip region.
Wind turbine blades have generally in the root region profiles of higher relative thickness than in the rest of the blade. An example of a root region profile having a relative thickness in the range 24%-26% is disclosed in EP 0 663 527 A1.
A profile in the root region of the blade is desired to have a high maximum lift coefficient and a high lift-to-drag ratio close to the maximum lift coefficient to aid rotor start-up and energy production at medium wind speeds. However, the value of the lift coefficient is usually moderate in this type of profiles and occurs at moderate low angles of attack. These effects result on the necessity of constructing the root area with high chords and high torsion (or twist) in order to obtain the maximum energy. However the manufacturing process is limited to certain values in chords and twist, so values for these variables are therefore restricted in order to optimize overall costs.
It is also desirable that those profiles be less sensitive to soiling and roughness effects, avoiding as much as possible losses of lift when external particles (for example due to ice or dirtiness) are deposited on the external surface, so they should be ideally designed for inducing a transition from laminar to turbulent flow near the leading edge.
In this respect, the paper AIAA-2003-0350, “Roughness Sensitivity considerations for tic root blade airfoils”, suggest the DU family of airfoils, having a relative high thickness to deal with the roughness sensitivity problems.
None of known designs produces fully satisfactory results, therefore a continuing need exists for wind turbine blades with an aerodynamic optimised profile in the root region.